- Returning `IntoElement` because it's more universal than `Element` and
allows us to easily return components.
- Using `impl IntoElement` in `RenderOnce` for consistency, which
requires `Component` to allocate an `AnyElement`. We use a bump
allocated arena anyway for these, and my benchmark doesn't show an
impact.

In this histogram, frames-0 is this PR, frames-1 is main.
Release Notes:
- N/A